The present invention relates to a pre-heating device for a nozzle for use in a melt prilling apparatus.
Various prilling processes have been proposed for making prills from various substances in order to prevent dust formation and improve flowability thereof. Among these, the most important process is the melt prilling process in which a melted substance is showered or sprayed from the top of a tower with a perforated nozzle or with a rotary disc for making particles by centrifugal force, and prills are obtained either by drying or solidifying the dropping liquid particles with countercurrent gas introduced from the bottom of tower, or by dropping said liquid particles into a medium which is inert to said melted substance.
In such a prilling process, smooth passage of the melted substance through the nozzle is most important. In this connection pre-heating of the nozzle is indispensable, particularly important so as to prevent viscosity increase of the melted substance in the vicinity of the nozzle at the start of the prilling process.
In the absence of pre-heating, or in case of insufficient pre-heating, the melted substance present in the vicinity of the nozzle cools to show a viscosity increase or eventually solidifies and clogs the nozzle thereby rendering the operation impossible. In such case, the substance, once having solidified in the nozzle, is not easily melted even by forced heating, and the nozzle has to be removed for cleaning or replaced by a new nozzle followed by resumed pre-heating before the prilling operation can be started. Also, even if the nozzle is not clogged completely, partial clogging of the nozzle will result in formation of relatively large dripping liquid particles which drop in an unsolidified state and stick to the bottom of tower. The presence of such sticking unsolidified substance will cause reheating, by the heat accumulated therein, of other particles completely solidified in the fall, thereby resoftening such particles and thus forming a deposit on the bottom of the tower, rendering discharge of the prills difficult. It therefore becomes necessary to interrupt the operation and remove the deposit formed on the bottom of the tower. Such deposit, being a defective product, not only reduces the yield or production but also hinders continuous operation.
In general the nozzle is mounted at the lower portion of the spray chamber which is provided with a jacket for heating with steam or a heating medium to heat and maintain the temperature of said chamber, but such heating is hardly capable of heating the nozzle. However, as explained in the foregoing description a complete nozzle pre-heating is indispensable.
For such nozzle pre-heating there are already known methods, such as disclosed in the Japanese Patent Publication Sho-37-5002, of externally pre-heating a nozzle fixed in the center of the tower with a lamp or a resistor type heater, or a method of preheating the nozzle externally within the tower by blowing heated air. However these pre-heating methods, all depending on external heating and mostly utilize radiant heating and thus of low heating efficiency which is further reduced by air flow from the bottom of the tower, were unsatisfactory for sufficient heating even after a prolonged pre-heating time.